The present invention is in an apparatus for cleaning sulphur and nitrogen containing flue gases.
Such a process is especially provided for use with so-called large boiler plants such as in power plants, to reduce the emission of dangerous oxygen compounds, such as sulphur (SO.sub.x), to a minimum and to comply with environmental protection requirements.
From the pamphlet "Electron Beam Flue Gas Cleanup Process" of the firm Ebara/Japan a process of the above described type is already known in which an approximately stoichiometric quantity of ammonia gas (NH.sub.3) is added to the flue gas before the reactive mixture is subjected to the effect of electron beams. Thereby the harmful components of the flue gas are converted to ammonium sulphate or ammonium nitrate, which are both suitable for use as fertilizer in agriculture.
The problem is experienced therein of introducing the electron beams, whose source--as a rule an electron-emitting cathode heated up to a high temperature--can only operate under high vacuum, into the reaction chamber, whose contents are at or above atmospheric pressure. Electron beams have the property of spreading out or diverging in a gaseous atmosphere through collisions with the gas molecules, so that their range after emerging from the vacuum is already considerably reduced at atmospheric pressure. Thus a large proportion of electron beam treatment processes are carried out under high or at least partial vacuum. A further problem arises in bringing about a desirably homogeneous interaction between the reaction mixture on the one hand and the electron beams on the other.
In the known process the electron beams are produced with an accelerating potential of 300 kV and injected into the reaction chamber through a "window" comprising a thin metal foil (Ti-Pd). The foil-window forms a barrier between the electron beam source, which is under high vacuum on the one hand, and the atmosphere in the reaction chamber on the other, but it is of sufficiently thin formation to be at least partially transparent to electron beams. The electron beams are periodically deflected over the window area by a special deflection system attached to the electron beam source so that the foil-window is not damaged by the beam's high energy density.
Such a process and the apparatus required for it have numerous disadvantages: the accelerating potential of 300 kV necessary to penetrate the foil-window requires an expensively constructed electron beam gun on account of the necessary insulation requirements. At the same time such a high accelerating potential produces strong X-rays at the point of incidence, so that extensive radiation shielding is required. Consequently, the foil-windows have a high transmission loss of about 25% which, with the necessary high usage of electricity, leads not only to correspondingly high losses but also to an extraordinarily high thermal loading on the window. Another consequence is that because of the interaction of the foil-window with the contents of the reaction chamber, it is necessary to replace the window frequently, so that a relatively large proportion of downtime of the whole cleaning plant is experienced. Thus, when a large boiler plant is in continuous operation, a plurality of cleaning plants, in parallel connection, must be provided.
It is on that account proposed in DE-OS No. 3501158 that, instead of electron beam guns (300-800 kV accelerating potential) provided with windows, use be made of such guns in which the electron beam passes through pressure step stages from high vacuum--in which it is produced--into the reaction chamber which stands at normal pressure (about 200 kV--accelerating potential). Such arrangements bring, on account of dispensing with the window, the advantage of simpler shielding against the X-rays produced by interaction of the electron beam with the material and escaping from the arrangement, and opening up also, for the increase of productivity, simpler and less costly technical possibilities. The proposal includes also arrangements as to how the electron beams can be brought into the reaction chamber and steered by electromagnetic control equipment in the reaction zone and dynamically distributed.